These glass-backed holders accept up to three commercially available cut-to-size gelatin filters for rear-insertion lenses. For the EF 300mm f/2.8L IS II USM Lens and EF 400mm f/2.8L IS II USM Lens.

The black 52mm Drop-in Gelatin Filter Holder from Canon is a convenient way to add filtration to your shots. This 52mm rear mounted holder contains accepts user-supplied cut gelatin filters. The filters are held in place via a pressure clip held firmly against the holder's frame. Up to three gel filters can be used within the holder.